1. Field of the Invention
The present invention relates to martensitic stainless steel product (in forms such as steel pipes, steel forgings, steel bars, and steel sheets) which contains Cr in an amount of 9 to 16 wt. % and which is suitably used as structural material for chemical plants as well as for oil wells and gas wells (hereinafter collectively called "oil wells") and pipelines thereof. In particular, the present invention relates to martensitic stainless steel product which has oxide scale layers and which exhibits excellent surface properties and high corrosion resistance, and to a production method therefor.
2. Description of the Related Art
Examples of steel used in oil wells include seamless steel pipe and welded steel pipe, which are also referred to as oil country tubular goods or line pipe. Generally, seamless steel pipe is manufactured through a hot-rolling pipe-making method as described below.
A billet serving as raw material is heated to about 1100 to 1300.degree. C., and subjected to piercing by use of a piercing mill of a skew-roll type (Mannesmann piercing mill), to thereby obtain a hollow shell. Subsequently, the hollow shell is subjected to elongation processing. Any of a variety of mills may be employed as the elongation mill used in elongation, and in particular a mandrel mill (Mannesmann-mandrel mill) is widely employed, as it provides excellent dimensional accuracy and productivity.
The above-mentioned mandrel mill elongates a hollow shell by means of a mandrel bar which has a lubricant for hot rolling applied on its surface and which is inserted into the hollow shell. The temperature of the hollow shell under elongation processing is normally about 1050 to 1200.degree. C. as measured at the entrance of the mill, and about 800 to 1000.degree. C. as measured at the exit of the mill.
The pipe hot-rolled by a mandrel mill is generally called pipe for finish rolling. The pipe for finish rolling is reheated to about 850 to 1100.degree. C. in a reheating furnace as needed, and finished by use of a finish rolling mill such as a stretch reducing mill or a sizing mill at a finish temperature of about 800 to 1000.degree. C., to thereby obtain a pipe of a predetermined product size.
Also, seamless steel pipe may be manufactured through a hot-extrusion pipe-making method represented by the Ugine Sejournet method and a hot-push pipe-making method represented by the Ehrhardt push bench method. In this case, after a seamless steel pipe undergoes hot-extrusion in a hot-extrusion pipe making method, a lubricant (generally a glass lubricant) is removed from the seamless steel pipe, and the pipe is then fed to the subsequent step. Also, after the pipe is subjected to hot-pushing in the hot-push pipe-making method, at least one of the inner surface and the outer surface of the seamless steel pipe is machined for reduction of eccentricity of its wall thickness, and the pipe is then fed to a subsequent step.
In contrast, welded steel pipe is manufactured from hoop steel or plate steel through a pipe-making method such as an ERW (electric-resistance-welding) pipe-making method, a TIG (Tungsten Inert Gas) welding pipe-making method, a laser welding pipe-making method, or a UO (UO press-forming)-SAW (Submerged Arc Welding) pipe-making method, to thereby obtain a pipe of a predetermined product size, followed by a subsequent step.
The thus-finished seamless steel pipe or welded steel pipe of predetermined product size is fed to a subsequent finishing step, in which the pipe is generally subjected to a heat-treatment for imparting a predetermined strength. Specifically, steel pipe manufactured from martensitic stainless steel containing Cr in the amount of 9 to 16 wt. % (hereinafter called simply "martensitic stainless steel") is subjected to a heat-treatment including the steps of reheating to 900.degree. C. or more, quenching, and tempering at 600 to 750.degree. C.
Subsequently, the thus-heat-treated martensitic stainless steel pipe is generally subjected to a descaling step comprising pickling or shot blasting, a straightening step performed by use of a straightening mill such as a rotary straightener, and a non-destructive testing step executed through visual check or ultrasonic flaw detection. The pipe is then shipped as is or after application of a rust-inhibiting oil on the inner and outer surfaces thereof.
The descaling step comprising pickling or shot blasting of the heat-treated martensitic stainless steel aims at removal of oxide scale (hereinafter called simply "scale") which has inevitably been formed on the inner and outer surfaces due to heating to 1300 to 1600.degree. C. in the preceding step.
If the scale formed on the inner and outer surfaces is partially peeled off during a straightening step, a testing step (including temporary storage), or transportation after shipping, the resultant unevenness on the pipe surfaces not only impairs product appearance, but also lowers the accuracy of a non-destructive test. In the worst case, the non-destructive test itself may become impossible to perform. Also, in application of a rust-inhibiting oil, such unevenness leads to nonuniform thickness of the applied oil.
In addition, if scale is peeled off during transportation after shipping, rust forms at the peeled-off portion. Further, in the case where such a product is used as oil country tubular goods or line pipe, the peeled-off portion becomes susceptible to pitting corrosion.
However, a descaling processing comprising pickling or shot blasting requires many steps and great cost, which leads to a decrease in productivity, an increase in production cost, and environmental pollution due to employment of a large amount of pickling liquid or shot blasting grains. For this reason, in recent years, consideration has been given to simplification of a descaling processing, as well as to shipment of steel pipe having scale that has not be subjected to the descaling processing.
On the surfaces of martensitic stainless steel pipe manufactured through a conventional method, two layers, i.e., an inner scale layer and an outer scale layer (hereinafter collectively referred to as a "dual scale layer"), are formed at the termination of heat-treatment. The outer and inner scale layers are relatively large in thickness, at about 70 .mu.m and about 50 .mu.m respectively, and poor in adhesion. Therefore, the steel pipe has a disadvantage in that a descaling step cannot be omitted in the manufacture thereof.
The inner scale layer is an oxide layer containing FeCr.sub.2 O.sub.4 in an amount of about 35 vol. % with the remainder substantially made up of Fe.sub.3 O.sub.4 or FeO as a main component. The outer scale layer is an oxide layer which, when FeCr.sub.2 O.sub.4 and Fe.sub.3 O.sub.4 are the main components of the inner scale layer, contains Fe.sub.3 O.sub.4 in an amount of about 80 vol. %, and when FeCr.sub.2 O.sub.4 and FeO are the main components of the inner scale layer, contains FeO in the amount of about 60 vol. % and Fe.sub.3 O.sub.4 in the amount of about 25 vol. %, with the remainder substantially made up of Fe.sub.2 O.sub.3. Also, the outer scale layer has a surface of Fe.sub.2 O.sub.3.
In some cases, the scale contains a trace amount of spinel oxides such as Fe.sub.2 SiO.sub.4 and FeO.Mn.sub.2 O.sub.3 in addition to the above-mentioned oxides.
Since the corrosion resistance of a product having scale during use as oil country tubular goods or line pipe has not yet been investigated, the corrosion mechanism and corrosion resistance (corrosion resistance to carbon dioxide gas, as well as resistance to localized corrosion and resistance to sulfide stress cracking in an atmosphere containing hydrogen sulfide) over long-term use remains unknown. Therefore, the product having scale involves a disadvantage in that a descaling step cannot be omitted.
The reason why the dual scale layer on the inner pipe surface are thicker than the dual scale layer on the outer pipe surface as mentioned above is that the atmospheric gas (air) contacting the inner pipe surface circulates more slowly than does that contacting the outer pipe surface.
Japanese Patent Application Laid-Open (kokai) No. 57-19329 discloses a method of controlling the scale formed on stainless steel product, in which scale is removed from the surfaces of a steel plate prior to quenching.
However, since this method employs a descaling step comprising long-time pickling or grinding outside an assembly line, insertion of the descaling step into a line of steps arranged in a continuous manner is difficult. Therefore, in practice this method cannot be applied to manufacture of seamless stainless steel where material is processed through respective steps in a short time.